Sorry for the misleading title; I have a new board that fits nicely on my roof rack upside down with the fin facing the rear of the car, but have been told that it should always go fin forward. To me, this sounds illogical as the fin acts as a wind vane and would tend to keep the board centered if in the rear and might push the board to one side or other if in the front (both scenarios assuming the straps/tie downs loosen up). Also, it looks cooler fin aft.
Why fin front? Thanks
Ps I keep a safety line around the fin, tied to rear strap, to keep the board from possibly sliding aft.

The reason the fin side goes to the front of the car is because the board rocker is almost flat on the back of the board and it won't push the board down or even push to board up off the racks.
You should never leave a fin on because you can hit something and bust your box or maybe even the board. Every time I see a guy with the board like you like to do it I just laugh because we know is a beginner. New comers are always welcome. Enjoy windsurfing no matter how you put your board in the racks but not fin back and even better with no fin.

This question of how to position a board on the car roof rack crossed my mind too. I have seen many people putting boards upside down with the front of the board pointing to the front of the car. It may look good as the board's shape follows the curve of a car's roof. But I think this will actually cause more drag hence you will get less milage per litre of gas. If we were to think of the aerodynamics of a board, then a board on the car roof would behave more like a spoiler if the board is placed with the tail of the board pointing to the front of the car and with the deck of the board facing up. Another reason why I place my board in this manner is to prevent possible damage to the bottom of the board.

One thing I don't do is to leave the fin still fixed on the board when I transport my board on a car roof rack.

Any one with a good understanding of aerodynamics care to share your thoughts on this?

Most of the time, I carry my board upside, front to the front, with the fin back. I even leave the mastfoot! I have to admit that I'm lazy and that I don't need to go far.

If I travel further, I will remove the fin (and mastfoot) to reduce drag. I put the board upside down, but still with the nose to the front. My board curve follows roughly the curved flow of air around the car. However, since the nose is larger than the tail, It might be better with the tail first? I'll try it next time.

With the tail first and upside down, the board is more stable with less frontal drag, no downward force, plus the nose rocker at the back directs the air flow down the back window and minimizes turbulance for better gas milage.

I have a van now, but traveled extensively at one time with up to three boards stacked in one pile on top of my sedan.

As far as I can see at least 90% of riders here in Europe travels with nose forward, boards upside down. In german highways I've travelled also for two hours at 170 kmh with Ventura 343-255 lt and over it Mistral Score 112.

One other tidbit for those that travel with boards on top. - ALWAYS use two straps on the front, one to secure the board(s) and the other for a back up. Generally, new straps will not fail, but as they age and get cooked by the sun, they begin to wear.

On a long trip at 70 mph with two boards on top, I left the highway on to a side street where the front strap suddenly broke, luckily while moving at only 30 mph. The boards shifted to the side, but did not fall off. From that point on, I always used a second "safety" strap on the front.

I shutter to think what may have happened if the strap broke 5 minutes earlier on a highway filled with cars.

Assuming the board is not in a board bag and it is placed upside down on the car roof rack, there will be a pressure differential when air passes over and under the board, hence there will be a lift force in the direction of the hull (is it called hull?, the bottom of the board).

With the nose pointing to the front, there will be more drag than if the tail is pointing to the front.

However, if the board is placed with the deck facing up and nose pointing to the back, the pressure differential will press the board down onto the roof rack, hence less force on the straps (lesser risk of strap breaking?)

2 other disadvantages of putting the board upside down:

a. the roof strap might cause depression marks on the hard edge of the board if straps are pulled very tight. (eg, slalom board)
b. risk of flying pebbles hitting and damaging the bottom of the board which you want to keep it smooth. (Deck of a board will be easier to repair if damaged)

However, I'm not sure if this drag & lift is significant with regards to fuel efficiency and when the board is transported with a boardbag. : )